This proposal describes preliminary results which indicate the feasibility of a new method of electrophilic labeling of peptides and other polar compounds with the positron-emitting isotope, 18F. It describes what further steps need to be taken to establish the method as a general, practical technique for labeling neuropeptides at levels of specific activity necessary for in vivo receptor mapping by positron emission tomography (PET), and outlines a series of simple experiments to that end. The method, which involves the reaction of acetyl hypofluorite directly with unprotected substrates in water, is applicable not only to peptides and proteins (e.g., antibodies) but also to a large variety of polar compounds which are difficult to label for PET by existing procedures.